1. Field of the Invention
The present invention relates to blasting caps, and more particularly to blasting caps which function in air but are inoperative in liquids such as water and oil. The invention relates also to a heat-sensitive exothermic-burning composition useful as an ignition charge for blasting caps.
2. Description of the Prior Art
U.S. Pat. Nos. 2,739,535, 2,759,417, and 2,891,477 describe the use of lined shaped charges of high-velocity detonating explosive to perforate oil well casings and the walls of oil wells. The shaped charges are mounted in a reusable perforating gun, and ordinarily are initiated by detonating cord, which in turn is initiated by a blasting cap, usually electric, located in the carrier gun. The lined cavity of the shaped charges must be free of substantially incompressible material which would interfere with the "jetting" action required for effective penetration of the casing. If a liquid such as water or oil enters the gun and the charges are detonated while the space surrounding them is filled with the liquid, the perforating gun may become damaged and permanently wedged in the oil well casing. To prevent this from happening, initiators have been developed which are inactivated by the presence of a liquid and thereby will not permit initiation of the shaped charges in the event liquid of any kind enters the perforating gun. These initiators function on the "gap" principle, according to which a free space is provided between reactive components of a blasting cap or between the blasting cap and the detonating cord to be initiated thereby. The cap functions and the detonation is transmitted to the cord when the space is filled with air but not when it is filled with liquid.
In the blasting caps described in U.S. Pat. Nos. 2,739,535 and 2,759,417, the ignition charge is separated from the priming charge, and holes are present in the wall of the cap shell around the free space between these charges. The cap is located at the bottom of the perforating gun barrel, and liquids that leak into the barrel surround the cap and enter the holes therein, thereby preventing the ignition impulse from reaching the priming charge and inactivating the cap. One of the drawbacks of this cap is that loose live powder can fall out of the holes during manufacture and handling unless the powder is introduced as preformed pellets.
In the device shown in U.S. Pat. No. 2,891,477, a blasting cap is maintained in axial alignment with a receptor shell containing an impact-sensitive explosive charge, the closed, base-charge end of the cap facing, but being spaced apart from, the closed end of the receptor shell. When the space between shells is empty, the impact-sensitive receptor charge is initiated over the air gap by the detonation of the cap's base charge, and the receptor charge (or a booster charge adjacent thereto) initiates a detonating cord fitted into the receptor's opposite end. The means used to maintain the positioning of the cap and the receptor shell has openings in the area around the gap to permit the entry of liquids. When a liquid is present in the gap, the cap may detonate but it cannot initiate the cord because the receptor charge does not detonate. This design avoids the problem of loose powder falling out of holes surrounding the gap, but suffers from certain other disadvantages common to the "gap-type" caps. Some liquid-disabled caps of this design might be mounted in a small-diameter pipe. Such caps might detonate in liquids because the shock wave is guided by the pipe to the receptor charge in almost undiminished strength. Another disadvantage is that the discontinuity in the reaction train introduced by the gap is a possible source of malfunction (i.e., failure of the reaction to propagate from one charge to the next) when the detonating cord and shaped charges are supposed to detonate, and, in any event, limits the choice of explosives that can be used with any given set of cap specifications. For example, in the device of U.S. Pat. No. 2,891,477, a change in base charge composition to comply with use requirements may necessitate a change in base charge size, gap size, and/or receptor charge. Also, the proper functioning of such devices is dependent on achieving a high degree of precision in manufacturing with respect to gap size, and uniformity of donor and receptor charge composition and size. Such precision is difficult to achieve with standard blasting cap loading machines. Still another disadvantage is the added length of the device in contrast to standard blasting caps.